1. Field of the Invention
The present invention relates to radiation therapy, and more specifically it relates to a method for modeling the precise microscopic interactions of electrons with matter to enhance physical understanding of radiation sciences.
2. Description of Related Art
Currently in the United States, radiation therapy is used to treat about 60% of all cancer patients. Since radiation therapy targets specific areas of the body, improvement in radiation treatment techniques has the potential to reduce both mortality and morbidity in a large number of patients.
The radiation source may be in the form of external beams of ionizing particles or radioactive sources internal to the patient. External beams are usually produced by machines acting as particle accelerators. The beam delivery system consists of the radiation source, which is mounted on a gantry which can rotate about a 360.degree. arc around the patient. Each beam is shaped by a rotatable collimator. The patient lies on a rotatable table. The gantry and table both rotate about a single isocenter.
External beam radiation therapy is performed with several types of ionizing radiation. Approximately 80% of patients are treated with photons, ranging in maximum energy from 250 keV to 25 MeV. The balance are treated primarily with electrons with energies from 4 to 25 MeV. In addition, there are several fast neutron and proton therapy facilities which have treated thousands of patients worldwide. Fast neutron therapy is performed with neutron energies up to 70 MeV, while proton therapy is performed with proton energies ranging from about 50 to 250 MeV. Boron neutron capture therapy is conducted with thermal and epithermal neutron sources. Most internal radioactive sources irradiate the patient with photons, although some sources emit low energy electrons.
The effects of ionizing radiation on the body are quantified as radiation dose. Absorbed radiation dose is defined as the ratio of energy deposited to unit mass of tissue. Because tumors and sensitive structures are often located in close proximity, accuracy in the calculation of dose distributions is critically important. The goal of radiation therapy is to deliver a lethal dose to the tumor while maintaining an acceptable dose level in surrounding sensitive structures. This goal is achieved by computer-aided planning of the radiation treatments to be delivered. The treatment planning process consists of characterizing the individual patient's anatomy (most often, this is done using a computed tomography (CT) scan), determining the shape, intensity, and positioning of radiation sources (the subject of the present invention), and calculating the distribution of absorbed radiation dose in the patient. Most current methods used to calculate dose in the body are based on dose measurements made in a water box. Heterogeneities such as bone and airways are treated in an approximate way or ignored altogether. Next to direct measurements, Monte Carlo transport is the most accurate method of determining dose distributions in heterogeneous media. In a Monte Carlo transport method, a computer is used to simulate the passage of particles through an object of interest.
The CREEP single scatter electron Monte Carlo code, the subject of this invention, is designed to be the first phase in a two-part approach to a advanced electron transport package for PEREGRINE. PEREGRINE is an all-particle, first-principles 3D Monte Carlo dose calculation system designed to serve as a dose calculation engine for clinical radiation therapy treatment planning (RTP) systems. By taking advantage of recent advances in low-cost computer commodity hardware, modem symmetric multiprocessor architectures and state-of-the-art Monte Carlo transport algorithms, PEREGRINE performs high-resolution, high accuracy, Monte Carlo RTP calculations in times that are reasonable for clinical use. Because of its speed and simple interface with conventional treatment planning systems, PEREGRINE brings Monte Carlo radiation transport calculations to the clinical RTP desktop environment. PEREGRINE is designed to calculate dose distributions for photon, electron, fast neutron and proton therapy.
The PEREGRINE Monte Carlo dose calculation process depends on four key elements: complete material composition description of the patient as a transport mesh, accurate characterization of the radiation source , first-principles particle transport algorithms (the subject of the present invention), and reliable, self-consistent particle-interaction databases (also an element of the present invention). PEREGRINE uses these elements to provide efficient, accurate Monte Carlo transport calculation for radiation therapy planning.
The patient transport mesh is a Cartesian map of material composition and density determined from the patient's CT scan. Each CT scan pixel is used to identify the atomic composition and density of a corresponding transport mesh voxel. Atomic composition is determined from CT threshold values set by the user or by default values based on user-specified CT numbers for air and water. The user also assigns materials and densities to the interior of contoured structures. If the user specifies a structure as the outer contour of the patient, PEREGRINE constructs a transport mesh that is limited to the maximum extent of that structure, and sets all voxels outside that structure to be air. This provides a simple method of subtracting the CT table from the calculation. The default resolution of the transport mesh is 1.times.1.times.3 mm, for small-volume areas such as the head and neck, or 2.times.2.times.10 mm, for large-volume treatment sites such as the chest and pelvis. The resolution can also be reduced from the CT scan resolution. For reduced-resolution voxels, material composition and density are determined as the average of all CT pixels that fall within the transport mesh voxel.
The PEREGRINE source model, designed to provide a compact, accurate representation of the radiation source, divides the beam-delivery system into two parts: an accelerator-specific upper portion and a treatment-specific lower part. The accelerator-specific upper portion, consisting of the electron target, flattening filter, primary collimator and monitor chamber is precharacterized based on the machine vendor's model-specific information. These precharacterized sources are derived from Monte Carlo simulations from off-line Monte Carlo simulations using BEAM and MCNP4A, as described in copending U.S. Pat. No. 5,870,697, which is fully incorporated herein by reference. Particle histories from off-line simulations are cast into multidimensional probability distributions, which are sampled during the PEREGRINE calculation. The photon beam is divided into three subsources: primary, scattered, and contaminant. Separating the source into subsources facilitates investigation of the contributions of each individual component. To ensure site-specific model accuracy, the installation procedures will consist of a limited number of beam description parameter adjustments, based on simple beam characterization measurements. The lower portion of the radiation source consists of treatment-specific beam modifiers such as collimators, apertures, blocks, and wedges. This portion is modeled explicitly during each PEREGRINE calculation. Particles are transported through this portion of the source using a pared-down transport scheme. Photons intersecting the collimator jaws are absorbed. Photons intersecting the block or wedge are tracked through the material using the same physical database and methods described below for patient transport. However, all electrons set in motion by photon interactions in the block or wedge are immediately absorbed.
Using the Monte Carlo transport method, PEREGRINE tracks all photons, electrons, positrons and their daughter products through the transport mesh until they reach a specified minimum tracking energy or leave the patient transport mesh. Developing good statistics requires tracking millions of representative particles (histories) through the patient transport mesh. During the simulation, PEREGRINE records energy deposited at each interaction, which builds up a map of energy deposited in the transport mesh. After the Monte Carlo process is finished, a dose map is created by dividing the total energy deposited in each voxel by its material mass. PEREGRINE transports photons through the body using the standard analog method. Woodcock or delta-scattering is used to efficiently track particles through the transport mesh. All photons below 0.1 keV energy are absorbed locally. PEREGRINE transports electrons and positrons using a class II condensed-history scheme. This procedure groups soft collisions with small energy losses or deflections, but simulates directly those major or catastrophic events in which the energy or deflection angle is changed by more than a preset threshold. Delta-ray and bremsstrahlung production are modeled discretely for energy transfers &gt;200 keV. PEREGRINE uses Moliere's theory of multiple elastic electron/positron scattering . Pathiength corrections described are used to account for the effect of multiple scattering on the actual distance traveled by the electron or positron. A minimum electron/positron transport energy is assigned to each transport voxel based on range rejection. The range-rejection minimum energy corresponds to the minimum electrort/positron range required to traverse 20% of the minimum zone dimension, with range determined as the minimum range calculated for that zone plus all directly adjacent zones. Two 511 keV photons are created at the end of each positron range. The direction of the first photon is chosen randomly, while the second is set to 180.degree. opposed to the first.
The accuracy of Monte Carlo dose calculations depends on the availability of reliable, physically-consistent physical databases. For photon/electron/positron transport, PEREGRINE relies on the Lawrence Livermore National Laboratory Evaluated Physical Database, combined with stopping powers supplied by the National Institute of Standards and Technology. CREEP uses the LLNL Evaluated Electron Data Library, which is described in further detail below.
PEREGRINE accounts for photon interactions via the photoelectric effect, incoherent/coherent photon scattering, and pair production. All photon cross sections used by PEREGRINE are derived from the Lawrence Livernore National Laboratory Evaluated Photon Data Library (EPDL). EPDL data are taken from a variety of sources that have been selected for accuracy and consistency over a wide range of photon energies (10-eV-100-MeV) for all elements.
At low incident photon energies (&lt;0.1 MeV for tissue components, &lt;1 MeV for high-Z materials such as lead and tungsten), the photoelectric effect is the dominant absorption mechanism. The cross sections contained in PEREGRINE were obtained by direct evaluation of the relativistic S-matrix in a screened central potential. These cross sections accurately describe ionization from electrons bound in isolated atoms and provide predictions at the percent level for compounds where the K and L shells are well-represented by atomic orbitals. For most elements, at energies typical of those encountered for clinical photon beams, Compton scattering is the most important process in the photon-atom interaction. The Compton scattering cross sections used in PEREGRINE are obtained in the incoherent scattering factor approximation. This approximation includes screening effects. Relativistic effects enter through use of the Klein-Nishina cross section. Coherent scattering does not contribute significantly to the total photon-atom interaction cross section for most radiation therapy applications. However, these cross sections are still modeled, and were obtained under similar assumptions to those for incoherent scattering. At very high incident photon energies (&gt;30 MeV for tissue components, &gt;5 MeV for high-Z materials such as lead and tungsten), the dominant photon interaction mechanism is pair production. The cross sections for pair and triplet production used by PEREGRINE include Coulomb and screening effects and radiative corrections.
PEREGRINE accounts for the effects of large-angle elastic scattering (delta-ray production) and bremsstrahlung production on an event-by-event basis. All other energy-loss mechanisms are accounted for through continuous-slowing-down-approximation (CSDA) energy loss.
Moller (Bhabha) scattering is the ionization of an atom by an electron (positron). Moller and Bhabha cross sections and sampling methods follow those given by Messel and Crawford. The threshold for these processes in PEREGRINE is set so that the ejected electron kinetic energy is &gt;200 keV.
Bremsstrahlung cross sections contained in PEREGRINE are derived from the LLNL Evaluated Electron Data Library (EEDL). These cross sections were determined by interpolating between the relativistic S-matrix data available up to 2 MeV, and the Bethe-Heitler result, expected to be valid above 50 MeV. Bremsstrahlung cross sections are processed to reflect a bremsstrahlung photon energy cutoff of 200 keV. During transport, PEREGRINE uses restricted collision and radiative stopping powers, which exclude energy lost due to Moller/Bhabha events with energy transfers &gt;200 keV and bremsstrahhrng events with energy transfers &gt;200 keV. Restricted collision stopping powers are calculated. Restricted radiative stopping powers are calculated by subtracting the total energy transferred to the bremsstrahlung photon per distance, as determined from the bremsstrahlung cross section data.
The accuracy of PEREGRINE transport calculations has been demonstrated by benchmarking PEREGRINE against a wide range of measurements and well-established Monte Carlo codes such as EGS4 and MCNP. The accuracy of the CREEP-based electron transport package has been demonstrated by comparing PEREGRINE with and without this feature enabled, as well as comparing CREEP results to calorimetric experiments directly, independent of the PEREGRINE code.